Flexography was the name originally adopted for use in the packaging industry as applied to direct rotary printing using a raised image on a flexible printing plate adhered to a plate cylinder or printing roller. Additionally flexible plates have been adapted to letterpress and letterset printing where the plates can be suitably inked for carrying a fluid or paste ink to virtually any substrate. Rubber plates were used in the beginning and more recently along with rubber plates other flexible elastomeric materials have been made available for use in photopolymer plate or flexographic systems.
Currently certain flexible printing plates are made from various synthetic polymers that can be made light reactive so that a surface properly exposed can be etched to remove the non-printing areas in order to produce a plate having a cured or somewhat hardened raised printing surface. The flexible photopolymer surface is preferably backed with a dimensionally stabilizing polyester support or other sheet material that serves to reduce register distortions that might result from the processing, mounting, or usage of the plate.
Flexible rubber plates are made from a photographic negative by making a surface on a zinc plate light sensitive and then etching the zinc plate after exposure through the negative. The zinc plate is then impressed in a mold while heat and pressure is applied and the mold is used to produce a flexible rubber printing plate.
Such flexible plates produced from photographic negatives are currently used for black and white as well as color printing on paper, plastic film, metal foil, but they can also be used on most any other surface that can be run through a printing press. One of the requirements for high quality multi-color printing is that all of the printing surfaces on the respective color printing plates be properly positioned on their respective plate rollers so that when the web being printed upon is fed into contact with printing plates mounted on the successive plate rollers in the press, the several colors will be applied properly to the web or substrate to be precisely positioned in their respective zones relative to the other colored zones both horizontally and vertically. This is essential in order that the colors will be placed on the web in the desired exact position to form the composite images which together reproduce the original photograph being duplicated. This exact alignment of the plates that is necessary for printing of the several colors on the web is referred to as registration and also, to some extent, precise registration is a requirement in black and white printing in certain instances where it is necessary to coordinate the printed matter with structural features on the web, for example.
Various mechanical and optical methods have been developed to obtain the desired registration of the plates for printing on their webs. To one degree or another these prior art systems have not always been too satisfactory either because of the labor involved, the expense, and accuracy of the mounting of the flexible plates on their respective plate rolls. The present invention provides a combination of known technology used in a novel combination to overcome the deficiencies of the prior art and provides an improved and inexpensive, easy to use, procedure for precisely mounting flexible flexographic letterpress or letterset printing plates on their plate rollers. More particularly this invention provides a method of preparing the flexible printing plate itself, together with provision of a very simple device adapted to be used with the plate to ensure a precision mounting of a given plate or a series of color printing plates on their respective plate rollers, which device minimizes the degree of skill needed to produce proper registration of the printing plates.